The present invention relates capacitive micromachined ultrasonic transducers (cMUT), particularly to methods for operating cMUT.
Capacitive micromachined ultrasonic transducers (cMUTs) are electrostatic actuator/transducers, which are widely used in various applications. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. These transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and many others. In addition to discrete ultrasound transducers, ultrasound transducer arrays containing multiple transducers have been also developed. For example, two-dimensional arrays of ultrasound transducers are developed for imaging applications.
Compared to the widely used piezoelectric (PZT) ultrasound transducer, the MUT has advantages in device fabrication method, bandwidth and operation temperature. For example, making arrays of conventional PZT transducers involves dicing and connecting individual piezoelectric elements. This process is fraught with difficulties and high expenses, not to mention the large input impedance mismatch problem presented by such elements to transmit/receiving electronics. In comparison, the micromachining techniques used in fabricating MUTs are much more capable in making such arrays. In terms of performance, the MUT demonstrates a dynamic performance comparable to that of PZT transducers. For these reasons, the MUT is becoming an attractive alternative to the piezoelectric (PZT) ultrasound transducers.
The basic structure of a cMUT is a parallel plate capacitor with a rigid bottom electrode and a top electrode residing on or within a flexible membrane, which is used to transmit (TX) or detect (RX) an acoustic wave in an adjacent medium. A DC bias voltage is applied between the electrodes to deflect the membrane to an optimum position for cMUT operation, usually with the goal of maximizing sensitivity and bandwidth. During transmission an AC signal is applied to the transducer. The alternating electrostatic force between the top electrode and the bottom electrode actuates the membrane in order to deliver acoustic energy into the medium surrounding the cMUT. During reception the impinging acoustic wave vibrates the membrane, thus altering the capacitance between the two electrodes. An electronic circuit detects this capacitance change.
Two representative types of cMUT structures are conventional flexible membrane cMUT and the newer embedded-spring cMUT (EScMUT). FIG. 1 shows a schematic cross-sectional view of a conventional flexible membrane cMUT 10, which has a fixed substrate 101 having a bottom electrode 120, a flexible membrane 110 connected to the substrate 101 through membrane supports 130, and a movable top electrode 150. The flexible membrane 110 is spaced from the bottom electrode 120 by the membrane supports 130 to form a transducing space 160.
FIG. 2 is a schematic cross-sectional view of embedded-spring cMUT (EScMUT) 200, which is described in the PCT International Application No. PCT/IB2006/051568, entitled MICRO-ELECTRO-MECHANICAL TRANSDUCERS, filed on May 18, 2006; and International Application (PCT) No. PCT/IB2006/051569, entitled MICRO-ELECTRO-MECHANICAL TRANSDUCERS, filed on May 18, 2006, particularly the cMUTs shown in FIGS. 5A-5D therein. The cMUT 200 has a substrate 201, on top of which is a spring anchor 203 supporting a spring layer 210; a surface plate 240 connected to the spring layer 210 through spring-plate connectors 230; and a top electrode 250 connected to the surface plate 240. The cMUT 200 may be only a portion of a complete cMUT element (not shown).
Although structurally and mechanically very different, cMUTs 100 and 200 in FIGS. 1-2, and most other cMUTs, can be commonly represented by a simplified schematic model. FIG. 3A shows a simplified schematic cMUT model 300 which shows capacitor 310 consisting of fixed electrode 310a and movable electrode 310b, which is connected to equivalent springs 320 anchored by spring anchors 330. The fixed electrode 310a and the mobile electrode 310b define transducing space 360 therebetween. The electrodes 310a and 310b are connected to an interface circuit 380. The cMUT model can be further simplified as a circuit model having a variable capacitor as shown in FIG. 3B. The variable capacitor 310 in FIG. 3B has two electrodes 310a and 310b and is connected to the interface circuit 380.
Essentially all cMUTs based on a variable capacitor, even comb driver cMUTs in which the movable electrode is laterally displaced (along the direction of the electrode surface), may be represented by the variable capacitor model 300 shown in FIG. 3B. In this description, the variable capacitor model 300 shown in FIG. 3B is be used to represent any cMUT regardless of its structural and mechanical characteristics.
Usually a cMUT is biased with a DC voltage either directly or through a bias circuit. The cMUT also connects to an interface circuit, which usually comprises a switch, a transmission (TX) port and a reception (RX) port. In transmission, a transmission input signal is applied on the cMUT through the transmission port to move a movable electrode of the cMUT, which in turn energizes the medium and transmit acoustics energy into the medium. In reception, acoustic energy impinging on the cMUT is detected electrically by an interface circuit through the reception port. The switch switches the connection of the cMUT to either transmission port or reception port during operation.
One problem cMUT operation often has is the high-frequency harmonic distortion in the signal. The problem is explained as follows. If the transmission input signal is Vtx(t), and the DC bias is Vdc, the voltage V applied on the cMUT is V=Vdc+Vtx(t). The electrostatic force/pressure applied on the cMUT is proportional to V2:Pressure ∝ V2=(Vdc+Vtx)2=Vdc2+2VtxVdc+Vtx2.
In the existing cMUT operation methods, the linear term 2VtxVac is the wanted output pressure signal and the cMUT system and its operation are usually designed according to this premise. Vdc may be set to bias the cMUT to a desired operation point. The transmission input signal Vtx(t) is usually an AC signal which has a frequency matching the cMUT operating frequency ω:Vtx(t)=Vac sin(ωt)=(Vp-p/2)sin(ωt),
where Vac and Vp-p are the amplitude and peak-peak voltage of the applied AC signal, respectively; ω is the operation frequency of the cMUT. The electrostatic force/pressure is then expressed as:
      Pressure    ∝                  (                              V            dc                    +                      V            tx                          )            2        =            (                        V          dc          2                +                              V                          p              -              p                        2                    8                    )        +                  V                  p          -          p                    ⁢              V        dc            ⁢      sin      ⁢                          ⁢              (                  ω          ⁢                                          ⁢          t                )              -                            V                      p            -            p                    2                8            ⁢              cos        ⁡                  (                      2            ⁢                                                  ⁢            ω            ⁢                                                  ⁢            t                    )                    
where
  (            V      dc      2        +                  V                  p          -          p                2            8        )is a DC term used for DC bias, Vp-pVdc sin(ωt) is the wanted output pressure signal at the cMUT operating frequency ω, while
      -                  V                  p          -          p                2            8        ⁢      cos    ⁡          (              2        ⁢        ω        ⁢                                  ⁢        t            )      is the undesired second harmonic distortion. Because a large Vp-p is generally needed to generate a large enough output pressure, the existing actuation methods usually have a significant second harmonic that may distort the output pressure. For example, if Vp-p=2Vdc, then:
      Pressure    ∝                  (                              V            dc                    +                      V            tx                          )            2        =                    3        8            ⁢              V                  p          -          p                2              +                            V                      p            -            p                    2                2            ⁢              sin        ⁡                  (                      ω            ⁢                                                  ⁢            t                    )                      -                            V                      p            -            p                    2                8            ⁢              cos        ⁡                  (                      2            ⁢            ω            ⁢                                                  ⁢            t                    )                    
In this case, the second harmonic is comparable to the output pressure signal at the fundamental operating frequency ω. This type of distortion may make it difficult to use cMUT to do applications such as Tissue Harmonic Imaging (THI). Therefore, new method is needed to improve cMUT operation.